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Terminology Related To Nuts And Bolts

Anti-Friction Coating : AF coatings are dry lubricants consisting of suspensions of solid lubricants, such as graphite, PTFE or molydbenum disulphide of small particle size in a binder. Such coatings can be applied to fastener threads to replace metallic coatings such as zinc and cadmium and offer maintenance free permanent lubrication. By careful selection of the lubricants, AF coatings can be designed to meet specific applications. The coatings are permanently bonded to the metal surface and provide a lubricating film preventing direct metal-to-metal contact.

Anti-Seize Compound : An anti-seize compound is used on the threads of fasteners in some applications. The purpose of the compound depends upon the application. It can prevent galling of mating surfaces - such compounds are frequently used with stainless steel fasteners to prevent this effect from occurring. In some applications it is used to improve corrosion resistance to allow the parts to be subsequently dis-assembled Thirdly, it can provide a barrier to water penetration since the threads are sealed by use of the compound.

Allowance : An intentional clearance between internal or external thread and the design form of the thread when the thread form is on it's maximum metal condition. Not all classes of fit have an allowance. For metric threads the allowance is called the fundamental deviation.

Angle Controlled Tightening : A tightening procedure in which a fastener is first tightened by a pre-selected torque (called the snug torque) so that the clamped surfaces are pulled together, and then is further tightened by giving the nut an additional measured rotation. Frequently bolts are tightened beyond their yield point by this method in order to ensure that a precise preload is achieved. Bolts of short length can be elongated too much by this method and the bolt material must be sufficiently ductile to cater for the plastic deformation involved. Because of the bolt being tightened beyond yield, its re-use is limited.

Basic Thread Profile : This is the theoretical profile of external and internal threads with no manufacturing tolerance applied. BEARING STRESS The surface pressure acting on a joint face directly as a result of the force applied by a fastener.

Bihexagon Head :A bolt or screw whose cross section of its head is in the shape of a 12 pointed star.

Black Bolts and Nuts : The word black refers to the comparatively wider tolerances employed and not necessarily to the colour of the surface finish of the fastener.

Bolt : A bolt is the term used for a threaded fastener, with a head, designed to be used in conjunction with a nut.

Breakaway Torque : The torque necessary to put into reverse rotation a bolt that has not been tightened.

Breakloose Torque : The torque required to effect reverse rotation when a pre-stressed threaded assembly is loosened.

BSF : British Standard Fine. A thread form based upon the British Standard Whitworth form but with a finer thread (more threads per inch for a given diameter). This thread form was first introduced in 1908, the thread form is specified in BS 84: 1956.

BSW : British Standard Whitworth. A thread form developed by Sir Joseph Whitworth in 1841. The thread form has rounded roots and crests, the thread form is specified in BS 84: 1956. This thread form was superceded by the Unified thread in 1948 and then the metric thread form.

Cadmium Electroplating : Coating of threaded fasteners with cadmium can provide the parts with excellent corrosion resistance. The appearance of the coating is bright silver or yellow if subsequently passivated. The friction values associated with this coating are also comparatively low. A chromate conversion coating is frequently applied to the surface to improve corrosion resistance. Cadmium is not now frequently used because of the environmental and worker health problems associated with the coating process and should not be used in applications above 250C or when contact with food is possible.

Clamping Force :
The compressive force, which a fastener exerts on the joint.

Class of Fit : The Class of Fit is a measure of the degree of fit between mating internal and external threads. Three main Classes of Fit are defined for metric screw threads :

• Fine : This has a tolerance class of 5H for internal threads and 4h for external threads.
• Medium : This has a tolerance class of 6H for internal threads and 6g for external threads.
Coarse : This has a tolerance class of 7H for internal threads and 8g for external threads.

For Unified threads, a similar designation as for metric threads is used. The thread classes used are 1A, 2A and 3A for external threads and 1B, 2B and 3B for internal threads.

Coefficient of Friction : A dimensionless number representing the ratio of the friction force to normal force. Typically for threaded connections it is between 0.10 to 0,18 but can vary significantly depending upon the materials used and whether a lubricant has been used.

Design Form of Thread : The design form of an internal or external thread is the thread form in it's maximum metal condition. It is the same as the basic thread profile except that the thread roots are rounded. If either the internal or external thread form exceeds the design form of the thread profile then a potential interference exists.

Direct Tension Indicators : Direct Tension Indicators (DTI's) is a term sometimes used to describe load-indicating washers. Projections on the face of the washer (usually on the face abuting the bolt head or nut) that deform under loading as the bolt is tensioned. An indication of the tension in the bolt can be made by measuring the gap between the washer face and the nut or bolt head. The smaller the gap - the greater the tension in the bolt. Commonly used in civil rather than mechanical engineering applications.

Dynamic Friction : Resistance to relative movement of two bodies that are already in motion.

Effective Diameter : This is the diameter of an imaginary cylinder coaxial with the thread, which has equal metal and space widths. It is often referred to as pitch diameter. Sometimes referred to as the simple effective diameter to differentiate from the virtual effective diameter.

Effective Nut Diameter : Twice the effective nut radius.

Effective Nut Radius : The radius from the center of the nut to the point where the contact forces, generated when the nut is turned, can be considered to act.

Electroless Nickel : A relatively thin, hard coating that can be applied to threads and deposited uniformly. Bright metallic in appearance this coating has excellent resistance to wear and corrosion.

Embedment : Localized plastic deformation, which occurs in the vicinity of clamped fasteners or in the fastener threads.

External Force or Load : Forces exerted on a fastener as a result of an applied loading to the joint.

External Thread : A screw thread, which is formed on an external cylinder, such as on bolts, screws, studs etc.

Friction :Mechanical resistance to the relative movement of two surfaces. There are two main types of friction; STATIC FRICTION and DYNAMIC FRICTION. Typically static friction is greater than dynamic friction.

Galling :
A severe form of adhesive wear which occurs during sliding contact of one surface relative to another. Clumps of one part stick to the mating part and break away from the surface. (Can frequently occur when both the nut and bolt are zinc coated.)

Grip Length :
Total distance between the underside of the nut to the bearing face of the bolt head; includes washer, gasket thickness etc.

Hard Joint : A joint in which the plates and material between the nut and bolt bearing surfaces have a high stiffness when subjected to compression by the bolt load. A joint is usually defined as hard if the bolt is tightened to its full torque and it rotates through an angle of 30 degrees or less after it has been tightened to its snug condition.

Hardened Washers : The force under the head of a bolt or nut can exceed, at high preloads, the compressive yield strength of the clamped material. If this occurs excessive embedding and deformation can result in bolt preload loss. To overcome this hardened washers under the bolt head can be used to distribute the force over a wider area into the clamped material. A more modern alternative is to use a flange headed nuts and bolts.

Heat Tightening : Heat tightening utilises the thermal expansion characteristics of the bolt. The bolt is heated and expands: the nut is indexed (using the angle of turn method) and the system allowed to cool. As the bolt attempts to contract it is constrained longitudinally by the clamped material and a preload results. Methods of heating include direct flame, sheathed heating coil and carbon resistance elements. The process is slow, especially if the strain in the bolt is to be measured, since the system must return to ambient temperature for each measurement. This is not a widely used method and is generally used only on very large bolts.

Hot Bolting : This term is used for the completion of maintenance work on a bolted joint when the joint is under loading. This can involve the replacement of individual bolts. There are risks both to the joint itself and to health and safety associated with this technique.

Hyraulic Tensioner : A hydraulic tool used to tighten a fastener by stretching it rather than applying a large torque to the nut. After the fastener has been stretched, the nut is run down the thread to snug up with the joint, the hydraulically applied load is then removed resulting in tension being induced into the fastener.

Hydrogen Embrittlement : Steel fasteners exposed to hydrogen can fail prematurely at a stress level well below the materials yield strength. Hydrogen embrittlement occurs in fasteners usually as a result of the part being exposed to hydrogen at some time during its manufacturing process but it can also occur through in-service corrosion. Electroplating is generally considered to be a major cause of hydrogen absorption in steel fasteners due to the release of hydrogen during this process. Higher strength steels are more susceptible to hydrogen embrittlement than lower strength steels, however it is considered that there is no lower strength limit. As a rule of thumb, steels below Rockwell C 35 are considered to be far less susceptible. Tests such as the incremental load hydrogen embrittlement test can be completed to assess if hydrogen embrittlement is present in a batch of fasteners.

Impact Wrench : A wrench, usually powered by electricity or air, in which repeated blows from little hammers are used to generate torque to tighten fasteners. The torque applied to the fastener depends upon the time and the air pressure applied to the tool (for pneumatic wrenches). The torque applied by an impact wrench to a fastener is influenced by the joint stiffness.

Internal Thread : A screw thread which is formed in holes, such as in nuts.

K Factor : The factor in the torque tightening equation: T=KDF where T is the fastener tightening torque in Newton meters, D is the fastener diameter in meters, F is the fasteners preload in Newtons and K is a factor whose value is often taken as 0.2. The formula gives the approximate tightening torque for standard fasteners used under normal conditions.

Lefthand Thread : A screw thread that is screwed in by rotating counterclockwise.

Length of Engagement : The axial distance over which an external thread is in contact with an internal thread.

Lock Nut : There are two common usage's of this term:

• A nut which provides extra resistance to vibration loosening by either providing some form of prevailing torque, or, in free spinning nuts, by deforming and/or biting into mating parts when fully tightened.

• The term is sometimes used for thin (or jam) nuts used to lock a thicker nut. When used in this way the thin nut should be adjacent to the joint surface and tightened against the thick nut. If placed on top of the thick nut the thin nut would sustain loads it was not designed to sustain.

Major Diameter : This is the diameter of an imaginary cylinder parallel with the crests of the thread; in other words it is the distance from crest to crest for an external thread, or root to root for an internal thread.

Metal to Metal Contact Flange Joint : A flanged joint in which a gasket is compressed by bolts - the gasket being located in a recess within the joint so that it is compressed by the bolt loads until metal to metal contact occurs. Unlike the FLOATING TYPE FLANGE JOINT, for metal-to-metal type joints there are no standardised gasket factor definitions, test procedures, nor generally acknowledged calculation procedures available.

Molybdenum Disulphide : A solid lubricant that acts as a high pressure resistant film. Can be used by itself as a dry lubricant as well as in with other solid lubricants and in oils and greases. Used in threads, such lubricants act as a separating film to prevent corrosion formation on the thread surface (even under adverse temperature and environmental conditions) ensuring the release of the threaded connection.

Nominal Diameter : The diameter equal to the external diameter of the threads.

Nut Runner : A torque control fastener-tightening tool that is usually powered by compressed air. The design of the tool is such that attempts are made to ensure that the applied torque is independent of joint stiffness.

Nyloc Nut : A torque prevailing nut that uses a nylon patented insert to provide a locking feature. The nylon insert, it is claimed, helps to seal the bolt thread against seepage of water, oil, petrol, paraffin etc.

Octagon Head : A bolt or screw whose head cross-section is a regular polygon with 8 sides.

Pitch :
The nominal distance between two adjacent thread roots or crests.

Preload : The tension created in a fastener when first tightened. Reduces after a period of time due to embedding and other factors.

Prevailing Torque : The torque required to run a nut down a thread on certain types of nuts designed to resist vibration loosening. The resistance can be provided by a plastic insert or a noncircular head.

Proof Load : The proof load of a nut is the axially applied load the nut must withstand without thread stripping or rupture. The proof load of a bolt, screw or stud is the specified load the product must withstand without permanent set.

Property Class : A designation system, which defines the strength of a bolt or nut. For metric fasteners, property classes are designated by numbers where increasing numbers generally represent increasing tensile strengths. The designation symbol for bolts consists of two parts:

• The first numeral of a two digit symbol or the first two numerals of a three digit symbol approximates 1/100 of the minimum tensile strength in MPa.

• The last numeral approximates 1/10 of the ratio expressed as a percentage between minimum yield stress and minimum tensile stress. Hence a fastener with a property class of 8.8 has a minimum tensile strength of 800 MPa and a yield stress of 0.8x800=640 MPa.
  
  The designation system for metric nuts is a single or double-digit symbol. The numerals approximate 1/100 of the minimum tensile strength in MPa. For example a nut of property class 8 has a minimum tensile strength of 800 MPa. A bolt or screw of a particular property class should be assembled with the equivalent or higher property class of nut to ensure that thread stripping does not occur.

Prying : The amplification of an external force acting on a bolt by a lever action which can occur when that force is an eccentric tensile load.

Reduced Shank Bolt : A bolt whose shank diameter is smaller than the nominal diameter of the bolt (normally the shank diameter of such a bolt is approximately equal to the effective diameter of the thread).

Relaxation : The loss of clamping force in a bolt, which commonly occurs as a result of embedment. Can also be caused by gasket creep, differential temperature expansion or vibration loosening.

Righthand Thread : A screw thread that is screwed in by rotating clockwise. The majority of screw threads are right handed.

Rolled Thread : A thread formed by plastically deforming a blank rather than by cutting. The majority of standard fasteners have their threads formed by rolling. Most threads are rolled before any heat treatment operation. Significant improvements in fatigue life can be achieved by rolling the thread after heat treatment, this improvement is due to compressive stresses being induced in the roots of the thread.

Root Diameter : Identical to MINOR DIAMETER

Screw : A headed threaded fastener that is designed to be used in conjunction with a pre formed internal thread or alternatively forming its own thread. Historically, it was a threaded fastener with the thread running up to the head of the fastener that has no plain shank. However this definition has largely been superseded to avoid confusion over the difference between a bolt and a screw.


Screw Thread : A ridge of constant section, which is manufactured so that a helix is developed on the internal or external surface of a cylinder.

Set Screw :A set screw is a threaded fastener that is typically used to hold a sleeve, collar or gear on a shaft to prevent relative motion. It is a threaded member that normally does not have a head. Unlike most other threaded fasteners it is basically a compression device normally used to generate axial thrust. Various socket types are provided to allow the set screw to be rotated. These types include hexagon socket, fluted socket, screwdriver slot and square head. Various point designs are available (the part of the set screw that rotates against the shaft being secured) and include :

• Cup : Hollowed end, is the most commonly used point style. Used when the digging in of the point is not undesirable.

• Cone : Pointed end, this type generates the highest torsional holding power and is typically used for a permanent connection.

• Oval : Rounded end that is typically used when frequent adjustment is required. The oval end prevents/reduces indentation.

• Flat : Cause little damage to the shaft and is used when frequent adjustment is required.

• Dog : Flat end with the threads stopping short of the end with the end fitting into a hole.

Shank : That portion of a bolt between the head and the threaded portion.

Shoulder Screws : A threaded fastener with a plain, precision machined, shank that is used for location purposes. They are typically used for pulleys and linkages.

Snug Torque : The torque required to pull plates together so that direct contact occurs; often used in angle control tightening. The snug torque ensures that metal-to-metal contact occurs at all the interfaces within the joint. It is only at this point that the required angle of rotation starts in order that the bolt is tightened sufficiently. The snug torque is usually determined experimentally on the actual joint.

Snugging : The process of pulling parts of a joint together, most of the input turn during this process is absorbed in the joint with little tension being given to the bolt.

Socket Head Cap Screw : A screw with a round head, usually with a hexagon indentation in the head for tightening purposes. Used on machine parts and is typically made from high strength steel (grade 12.9 in metric).

Soft Joint : A joint in which the plates and material between the nut and bolt bearing surfaces have a low stiffness when subjected to compression by the bolt load. In such a joint, the bolt (or nut) typically has to be tightened by two or more complete turns, after it has been torqued to the snug condition, before the full tightening torque is achieved. Often the placement of a gasket in a joint results in a soft joint.

Static Friction : Friction at rest; a force is required to initiate relative movement between two bodies - static friction is the force that resists such relative movement.

Stiffnut : A term used to describe a lock nut which has a prevailing torque.

Strength Grade See Property Class Stress Area : The effective cross sectional area of a thread when subjected to a tensile force. It is based upon a diameter that is the mean of the pitch (or effective) and the minor (or root) diameters of the thread.

Structural Bolt : A structural bolt is a heavy hexagon head bolt having a controlled thread length intended for use in structural connections and assembly of such structures as buildings and bridges. The controlled thread length is to enable the thread to stop before the joint ply interface to improve the fastener's direct shear performance. This term is used in civil and structural engineering but is not frequently used in mechanical engineering.

Stud : A fastener that is threaded at both ends with an unthreaded shank in between. One end (which often has a thread tolerance which results in more thread interference) is secured into a tapped hole; the other is used with a nut.

Tension Washers : A general name given to spring washers, curved washers, Belleville washers and disc springs. This type of washer provides a relatively low stiffness (compared to the joint stiffness) and can be used to act as a spring take-up with a bolt to prevent movement between parts.

Thread Crest : The top part of the thread. For external threads, the crest is the region of the thread which is on it's outer surface, for internal threads it is the region which forms the inner diameter.

Thread Flank : The thread flanks join the thread roots to the crest.

Thread Height : This is the distance between the minor and major diameters of the thread measured radially.

Thread Length : Length the portion of the fastener with threads formed.

Thread Root : The thread root is the bottom of the thread, on external threads the roots are usually rounded so that fatigue performance is improved.

Thread Runout : The portion at the end of a threaded shank which is not cut or rolled to full depth, but which provides a transition between full depth threads and the fastener shank or head.

Tolerance Class : A combination of tolerance grade and a fundamental deviation that is given to an internal or external thread. A tolerance class for an internal thread when combined with the tolerance class for an external thread gives the class of fit for the mating threads.

Tolerance Grade : The difference between maximum and minimum metal conditions for a tolerance applied to a screw thread. For metric threads the tolerance grade is given a number.

Torque Multiplier : A gearbox used to increase the torque produced by a small hand wrench.

Torque Wrench : A manual wrench that incorporates a display, gauge or other method to indicate the amount of torque transferred to the nut or bolt.

U Bolt : A U shaped fastener threaded at both ends used primarily in suspension and related areas of vehicles.

Wire Thread Insert : A threaded insert that is typically used for tapped hole repair or to improve the thread stripping strength of softer metals such as zinc and aluminum. The inserts are assembled into a previously tapped hole using a special driving tool. A thread-locking compound is frequently used to secure the insert if the assembly is subject to vibration.

Yield Controlled Tightening : A fastener tightening method, which allows a fastener to be tightened to yield. The angle of rotation of the fastener is measured relative to the applied torque, yield being assessed when the slope of the relationship changes to below a certain value. Sometimes called joint controlled tightening.